Diff washers are 22x27x1.5 Alloy or you could use copper. https://www.ebay.co.uk/itm/5-Five-Oil-Sump-Washers-BMW-02-E6-E10-E21-E12-E24-22mm-Al-07119963355-SW22X5-/171177956301?hash=item27db0053cd
The alloy washers corrode over time and turn into lace so would not be reusable.
Filler and drain on diff are 14mm Hex, remove the filler plug first so you know you can refill it.
Gearbox filler and drain plugs are tapered threads and do not use washers. Again remove the filler plug first before the drain plug.